Better versoning in python

requirements.py

from subprocess import call
import sys

class Package:
    def __init__(self, name, min=None, max=None):
        self.name = name
        self.min = min
        self.max = max

    def spec(self):
        result = "{}".format(self.name)
        if self.max and self.min:
            result += ">={},<{}".format(self.min, self.max)

        return result

packages = [
    Package("requests", "2.9.1", "2.10.0"),
    Package("scrapy", "1.0.3", "1.1.0"),
    Package("sqlalchemy", "1.0.10", "1.1.0")
]

def install(upgrade=False):
    for package in packages:
        command = ['pip', 'install']
        if upgrade:
            command.append('--upgrade')

        command.append(package.spec())
        
        call(command)

def freeze():
    with open('requirements.txt', 'w') as outfile:
        call(['pip', 'freeze'], stdout=outfile)

def print_usage_and_exit():
    print('Usage `python {} (install|upgrade)`'.format(sys.argv[0]))
    exit(1)


if __name__ == '__main__':
    if len(sys.argv) < 2:
        print_usage_and_exit()
    
    command = sys.argv[1]
    
    if command != "install" and command != "upgrade":
        print_usage_and_exit()

    install(command == "upgrade")
    freeze()